Espectros direccionais das ondas marítimas do Asar do Envisat:sua utilização na avaliação do recurso energético

Tese de mestrado, Ciências Geofísicas (Oceanografia), 2008, Faculdade de Ciências, Universidade de Lisbo

Coastal wave field extraction using TerraSAR-X data

The main highlights of TerraSAR-X (TS-X) synthetic aperture radar (SAR) imagery are a higher resolution of up to 1 m, when compared to conventional C-band SAR data, and a reduction of nonlinear imaging effects of a moving target by lower platform altitude. Thus, ocean waves with wavelength <30  m are detectable. This makes TS-X particularly useful to observe coastal areas where complex bathymetry strongly impacts the approaching waves. TS-X images acquired in different coastal areas are presented, including three case studies at the German coast. Wave fields (significant wave height and peak wavelength) are derived from the TS-X imagery using the proposed XWAVE algorithm and compared not only to in situ buoy wave measurements but also to results of a high-resolution numerical wave model. The objective is to study the quality of significant wave height field estimation in the spatial domain in highly variable conditions, which are typically dominant in coastal areas. The results show that the empirical XWAVE algorithm allows estimating wave fields from TS-X data with high resolution, thus showing the spatial information on wave variations. Therefore, it is a new useful tool to characterize sea state in coastal areas by remote sensing

Sea State measurements using TerraSAR-X/TanDEM-X Data

The high-resolution synthetic aperture radar (SAR) satellite TerraSAR-X (TS-X), which was successfully launched in 2007 and its twin TanDEM-X (TD-X) launched in 2010, deliver radar images of up to 1m resolution over land and ocean, for scientific and commercial applications. This thesis focuses on the development of an algorithm to derive the significant wave height (H_s) from TS-X and TD-X data. The result is the empirical algorithm XWAVE that, in contrast to other existing methods, operates without the need of a priori information. It is based on correlations of different SAR derived parameters with in-situ sea state measurements taken by buoys. The data used consists of a set of 200 TS-X/TD-X VV polarized StripMap SAR images collocated with buoys in deep water (>100 m). From that data-set, 100 data pairs are used for tuning and the remaining 100 are taken for validation of the algorithm. The algorithm uses Fourier analyses of TS-X/TD-X scenes and extracts spectral parameters. For the deduction of H_s, these parameters are weighted with coefficients tuned to match the collocated in-situ buoy wave measurements. The algorithm takes into account the dependency of the spectral properties on incidence angle, sea surface wind speed and wave traveling propagation direction with respect to the satellite heading. The validation results of H_s derived by the XWAVE algorithm against in-situ wave measurements show a very good agreement with a correlation R of 91%, a scatter index Si of 0.21 and a bias of 0.01 m. This thesis includes case studies of TS-X/TD-X acquisitions taken at different wave conditions and incidence angles. The intermediate steps and final results of the XWAVE algorithm are compared in detail with corresponding buoy measurements. Additionally, case studies of TS-X imagery of coastal areas are presented. The capability of the developed algorithm XWAVE to capture and resolve high spatial variable sea state conditions is demonstrated.Die hochauflösenden SAR Sensoren der Zwillingssatelliten TS-X und TD-X, die in den Jahren 2007 bzw. 2010 gestartet wurden, liefern Radarbilder von bis zu 1m Auflösung der Land oder Meeresoberfläche für wissenschaftliche und kommerzielle Anwendungen. Die vorliegende Arbeit beschreibt die Entwicklung eines Algorithmus zur Ableitung der Signifikanten Wellenhöhe Hs, aus TS-X oder TD-X Daten. Das Resultat dieser Doktorarbeit ist der empirische Algorithmus XWAVE. Er arbeitet autark und benötigt im Gegensatz zu bisherigen Ansätzen keine a-priori Information über den Seegang. Die entwickelte Methodik beruht auf der Analyse von Korrelationen verschiedener aus SAR Daten abgeleiteter Parameter und in-situ Messungen des Seegangs durch Bojen. Die verwendeten Daten bestehen aus 200 TS-X/TD-X VV polarisierten StripMap Szenen, die über Bojen im Tiefwasser (> 100m) aufgenommen wurden. Der Datensatz wurde unterteilt in 100 Szenen zur Feinjustierung der Algorithmusparameter und 100 Szenen zur Validierung der Ergebnisse. Der Algorithmus verwendet als Methode die Fourier Analyse der SAR Bilder um eine Zahl von spektralen Parametern zu extrahieren. Zur Ableitung von Hs werden diese Parameter in Termen mit angepassten Koeffizienten gewichtet um die Werte der Bojenmessungen zu reproduzieren. Der Algorithmus berücksichtigt dabei die Abhängigkeit der spektralen Eigenschaften vom Radar Einfallswinkel, von der lokalen Windgeschwindigkeit und vom Winkel zwischen Wellenausbreitungsrichtung und Satellitenflugrichtung. Der Vergleich der mit XWAVE errechneten Hs des Validierungsdatensatzes mit in-situ Bojenmessungen zeigt eine sehr gute Übereinstimmung mit einer Korrelation R von 91%, einem Scatter-Index Si von 0.21 und einem Bias von 0.01m. Die Arbeit enthält Fallstudien von TS-X/TD-X Aufnahmen bei verschiedenen Seegangsbedingungen und Einfallswinkeln. Die einzelnen Zwischenschritte und Terme des XWAVE Algorithmus werden im Detail mit den entsprechenden Bojenmessungen verglichen

Extraction of wave field from TerraSAR-X data

The new X-band Synthetic Aperture Radar (SAR) onboard the satellite TerraSAR-X (TSX) was launched on June 15, 2007. Since then it has provided numerous high quality data over land and ocean operationally. In the present study, integral wave parameters and two-dimensional ocean wave spectra are extracted from TerraSAR-X data. The preliminary results are compared to numerical wave models results, coastal X-band marine radar WaMoS (Wave and Surface Current Monitoring System) data. We developed an empirical geophysical model function (XWAVE) to derive significant wave height from TerraSAR-X data without using prior information. The comparison shows that the empirical model can yield reliable sea state measurements up to 5 m significant wave height. The model can be used for the near real-time services to deliver sea state measurements from high resolution TerraSAR-X data. Cross spectra are extracted from TerraSAR-X SSC (Single look Slant range Complex) products to remove the 180° ambiguity for wave propagation direction. A positive peak in the imaginary part of the cross spectrum indicates the ocean wave traveling direction. Peak wavelength is determined as well from the cross spectrum and compared to the WaMoS marine radar at Ekofisk oil platform in the North Sea is chosen as a test site

TerraSAR-X/TanDEM-X sea state measurements using the XWAVE algorithm

The high resolution TerraSAR-X (TS-X) satellite, which was successfully launched in 2007 and its twin TanDEM-X (TD-X) launched in 2010, deliver high-quality radar images over land and ocean, for scientific and commercial applications. We present an empirical algorithm, called XWAVE, to derive Significant Wave Height

Sea State Measurements Using TerraSAR-X Data

Synthetic Aperture Radar (SAR) is a unique sensor able to provide two dimensional information on meteo-marine parameters of the ocean surface e.g. wind speed and wave height. We present an algorithm (XWAVE) to derived significant wave height (Hs from TerraSAR-X (TS-X) data without the need of a priori information. The algorithm is based on spectral analyses of TS-X data, using a Geophysical model function, the coefficients are fitted to collocated in-situ buoy measurements using the least squares method. Wind speed information provided by the wind speed estimation XMOD algorithm [1] is combined in the XWAVE algorithm as well. Sea state parameters derived from TS-X data using the proposed algorithm have been compared to in-situ buoy measurements. Additionally, the one dimensional spectra are compared to the wave spectrum provided by the one given by the NOOA buoys. The SAR wave peak direction is also compared to the wave peak direction given by directional buoys. Sea state properties in coastal areas change rapidly in the spatial domain. TS-X data are especially suitable for the study of sea state variability in coastal areas due to its high resolution. Coastal wave fields have been derived from TS-X data and verified with numerical wave model results and with in-situ buoy measurements for the collated point in the area of the North Sea

Sea state measurements from TS-X SAR data

An empirical algorithm XWAVE to derive significant wave height from high resolution TerraSAR-X (TS-X) and Tandem-X (TD-X) has been developed. The algorithm is created especially for spaceborne X-band SAR data without needing a priori information. TS-X scenes were acquired in the over buoys located at the coast of United States of America and Canada and including Hawaii islands. Integral wave parameters are estimated from TS-X and TD-X data by integration two-dimensional image spectra and using geophysical function, for which the parameters are calibrated. TS-X Stripmap images and in-situ buoy measurements data is used to tune geophysical model parameters. The tuning is carried out by comparisons of integral wave parameters as wave height, peak period and direction (1) and by the shape of 1-D in-situ measured spectra and 1-D integrated image spectra (2). The results show that the model can yield sea state measurements up to 5m significant wave height by TS-X SAR with an scatter index of 0.21 . The model can be used for the near real-time services to deliver sea state measurement

High resolution satellite measurements of coastal wind field and sea state

Methods to derive wind speed and the sea state by simple empirical models from SAR data are presented and applied for use in high resolution numerical modeling for coastal application. The new radar satellite TerraSAR-X (TS-X) images the sea surface with a high resolution up to 1m. So not only the wind information, integrated sea state parameters but also individual ocean waves with wavelengths down to 30m are detectable. Two-dimensional information of the ocean surface retrieved using TS-X data is validated for different oceanographic applications: derivation of fine resolved wind fields (XMOD algorithm) and integrated sea state parameters (XWAVE algorithm). Both algorithms are capable to take into account fine-scale effects in the coastal areas. The wind and sea state information retrieved from SAR data are applied as an input for a wave numerical spectral model (wind forcing and boundary condition) running at fine spatial horizontal resolution of 100m. Results are compared to collocated buoy measurements. Studies are carried out for varying wind speed and comparison against wave height, simulated using original TS-X derived wind, show sensitivity of waves on local wind variation and thus the importance of local wind effects on wave behaviour in coastal areas. Examples for the German Bight, North Sea are shown. The TS-X satellite scenes render well developed ocean wave patterns of developed swell at the sea surface. Refraction of individual long swell waves at a water depth shallower than about 70m is caused by the influence of underwater topography in coastal areas is imaged on the radar scenes. A technique was developed for tracking of wave rays depending on the change of swell wavelength and direction. We estimate the wave energy flux along the wave tracks from deep water to the coastline based on SAR information: wave height and wave length are derived from TS-X data

Sea State Variability and Coastal Interaction Processes Observed by High Resolution TerraSAR-X Satellite Radar Images

Methods to derive wind speed and the sea state from Synthetic Aperture Radar (SAR) satellite data with a high resolution are presented and applied for use in numerical modeling for coastal application. The wind and sea state information retrieved from SAR data are applied as input for a wave numerical spectral model (wind forcing and boundary condition of sea state) running at fine spatial horizontal resolution of 100m. Examples for the German Bight, North Sea are shown